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Sometime during the upgrade, you must have been asked to create an account, or to say who you were, or some such.
That would have been used to create an account on the new install of Windows.
(The previous user account should still be on the computer though. Check to make sure your CapsLock light is not On, if it is, turn it Off by pressing CapsLock once.)
Try, as username and password, whatever you would have entered when asked for a name, or an account name, on the new Windows install.